Circular dependencies in Templates - better solution?

I am a newbie to C++ (~1 Week experience) and I have a few months of
experience with object-oriented languages (Objective-C). I am
currently working just for fun on a particle system.

All the particles are controlled by a "server". The server performs
all kinds of operations on them (updating, drawing etc.). The
particles (my "clients") on the other hand need to retrieve once a
while some information from their server - they need to be hooked up
to it. This works perfectly alright as long as I only have one
server-class and one client-class. However if I want to add some
subclasses and make everything more customizable I run into severe
problems. The lack of run-time dynamism caused some problems on
creating the server/client class structure.
If I use a new client-class it will most likely need a new
server-class. However its superclass already inherits a connection to
the old server-class. I would need to re-implement all the member
functions for the new server-class. In a pure dynamic language
(Smalltalk/Objective-C) I could simply ignore the static type
checking. To avoid compiler warnings I would simply cast the
object-pointers when necessary (just for cosmetic reasons).
C++ would not be C++ if it wouldn't deliver a solution for this
dilemma - Templates. When I need a new client class, I just need to
pass a new type parameter. Nice.
And C++ wouldn't be C++ if no new problems would arise. I will
demonstrate it.
My server-class implementations:

You can see that every client gets a connection to its server, when it
is constructed. But now try to instantiate a server with a
client-class.

ServerSubclass<ClientSubclass<ServerSubclass<ClientSubclass...> > >

We get an infinite circular dependency between the template classes.
It is impossible to solve this circular dependency unless I have
missed something fundamental about C++ templates.
However there is a not so elegant way to do it with partially
resorting to traditional OO constructions. All I need to do is to
rewrite my client-subclass without templates.

This looks confusing and it is indeed. But it is now possible to
instantiate a server.

ServerSubclass<ClientSubclass> myServer();

However subclassing without a template is not only a way to solve the
circular-dependency-problem but also to prevent any further
subclassing. If I wanted to subclass my first-subclass with all its
member functions I would need to convert my first subclass to a
template class and then subclass it twice (once for the first and once
for the second non-template subclass).

Am I totally wrong? Have I totally missed something about OO design?
Is there a more convenient way of doing this in C++ ?

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ro86 wrote in news: in
comp.lang.c++:
>
> ServerSubclass<ClientSubclass<ServerSubclass<ClientSubclass...> > >
>
>
> We get an infinite circular dependency between the template classes.
> It is impossible to solve this circular dependency unless I have
> missed something fundamental about C++ templates.
> However there is a not so elegant way to do it with partially
> resorting to traditional OO constructions. All I need to do is to
> rewrite my client-subclass without templates.
>

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Guest

Ok, this one is not easy.
The server-class template assumes that you pass a class which is again
a template as parameter. Right?
> std::vector< GenericClient< ServerClass > > (
> 10, GenericClient< ServerClass >(this)

I think this is the tricky part. The compiler knows that the
GenericClient awaits another class as parameter. If we would write this
outside of our template we would have a circular (or recursive)
dependency. But now we can pass the template class for our client as
parameter and thus avoid the circular dependency.
> HTH.
Yes, it did
Thanks!

wrote in news: in
comp.lang.c++:
> Rob Williscroft wrote:
>> template < template <typename> class GenericClient >
>> class ServerClass
>> {
>> //...
>> }
>
> Ok, this one is not easy.
> The server-class template assumes that you pass a class which is again
> a template as parameter. Right?

The paramiter is template-template paramiter and it must
be passed a class-template that takes one type paramiter.
>
>> std::vector< GenericClient< ServerClass > > (
>> 10, GenericClient< ServerClass >(this)
>
> I think this is the tricky part. The compiler knows that the
> GenericClient awaits another class as parameter. If we would write this
> outside of our template we would have a circular (or recursive)
> dependency. But now we can pass the template class for our client as
> parameter and thus avoid the circular dependency.
>

Well the bit you quoted is inside a constructor, templates and there
members are only instantiated when they're needed. Before it
instantiates the constructor it will instantiate the class to do that
it needs to instantiate the data member:

std::vector< GenericClient<ServerClass> > clients;

to do that it may need to instantiate:

GenericClient<ServerClass>

Fortunatly this can be done without knowing anything about
ServerClass as neither:

ClientSubclass< ServerClass > or
ClientSubclass< ServerSubClass >

require knowing anything about ServerClass or ServerSubClass
as they only use a pointers to a ServerClass.

The terminoligy used to refere to this is "ClientSubclass can
be instantiated with an incomplete type", ServerSubClass (and
ServerClass) being the incomplete (because they're currently
being instatiated) types.

Also I noticed that the code I posted may not do precisley
what you wanted, as the ServerSubClass contains a std::vector
of ClientSubClass< ServerClass > where as IIUC you wanted
a vector of ClientSubClass< ServerSubClass >.

Here's the fixed code, it has an extra paramiter to ServerClass
to feed in the sub-class type and a static_cast to cast from the
base type to the derived type:

Guest

This looks like the final solution. The original implementation hooked
the ClientSubclass to the ServerClass up. All I now need to do is to
wrap my classes into a elegant typedef and I can go on.
This has been very helpful. Problems like these are the only way to
learn more about complex languages like C++. The concept of templates
is fascinating because its very flexible but offers very high
performance (A glimpse into the future of meta-programming?). Learning
by doing =)
Danke!

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